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logarithmic or equiangular spiral, appear to have suggested the idea, 

 that not only the boundary of the operculum, which measures the 

 sectional expansion of a shell, but also the spiral lines, which in 

 general are well marked both externally and internally in the shell 

 itself, are curves of this nature. 



From an examination of the spirals marked on opercula, it appears 

 that the increase of their substance takes place on one margin only; 

 the other margin still retaining the spiral form, and acquiring an iu- 

 crease of length by successive additions in the direction of the curve. 

 As in the logarithmic spiral the distances of successive spires, mea- 

 sured on the same radius vector produced from the pole, from 

 each other, are respectively in geometrical progression, if similar 

 distances between the successive whorls on the opercula of shells be 

 found to observe the same law, it will follow that these whorls must 

 have a similar form; and that such is the case, the author shows by a 

 variety of numerical results obtained by careful measurements on 

 three different opercula of shells of the order Turbo. That such is 

 the law of nature in the formation of this class of shells is rendered 

 probable by the instances adduced by the author, in which a con- 

 formity to this law is found to exist. 



From the known properties of the logarithmic spiral the author 

 concludes that the law of the geometrical description of turbinated 

 shells is, that they are generated by the revolution about a fixed 

 axis, (namely, the axis of the shell,) of a curve, which continually 

 varies its dimensions according to the law, that each linear incre- 

 ment shall vary as the existing dimensions of the line of which it is 

 the increment. If such be the law of nature, the whorls of the shell, 

 as well as the spires on the operculum, must have the form of the 

 logarithmic spiral ; and that this is likewise the case is sho"UTi by 

 the almost perfect accordance of numerical results, deduced from the 

 property of that curve, with those deduced from a great variety of 

 careful measurements made of the distances between successive 

 whorls on radii vectores drawn on shells of the Turbo diiplicatiis. 

 Turbo 'phasianus, Buccinum suhulatum, and in a fine section of a 

 Nautilus pompilius. The author further states that, besides the results 

 given in the paper,' a great number of measurements were similarly 

 made upon other shells of the genera Trochus, Stromhus, and MureXy 

 all confirmatory of the law in question. 



One of the interesting deductions which the author has derived 

 ^ from the prevalence of this law in the generation of the shells of a 

 large class of mollusca, is that a distinction maybe expected to arise 

 with regard to the growth of land and of aquatic shells, the latter 

 serving both as a habitation and as a float to the animal w^hich forms 

 it; and that, although the facility of varying its position at every 

 period of its growth may remain the same, it is necessary that the 

 enlargement of the capacity of the float should bear a constant ratio 

 to the corresponding increment' of its body ; a ratio which always 

 assigns a greater amount to the increment of the capacity of the shell 

 than to the corresponding increment of the bullv of the animal. 



Another conclusion deducible from the law of formation here con- 



